This invention is directed to a simple method of forming a protective, hydrophobic coating on an aluminum surface.
Many processes are presently available for forming protective coatings on aluminum surfaces, such as anodizing, plating, chemical conversion coating, painting and the like. The coatings resulting from these processes, although designed for long life, require extensive surface pretreatments and are quite expensive. However, frequently, only short-term protection is needed, e.g., to prevent water stain in the shipping or storing of semifabricated aluminum products, such as coiled sheet and the like. In some instances, such as in welding or adhesive bonding aluminum products, it is desirable to prevent the gradual buildup of thick natural oxide on the aluminum surface, because a thick natural oxide coating can interfere with these processes. However, no simple and inexpensive process is presently known which will give short-term protection to aluminum products without interfering with subsequent fabrication, particularly when lubricants must be applied to the surface, or without detrimentally affecting the surface appearance of the fabricated product.
A simple, inexpensive process for coating aluminum is described by Wittrock et al in U.S. Pat. No. 3,726,721, assigned to the present assignee, but the friable coating formed by this process can have detrimental effects on the surface of the metal during subsequent forming or coating operations.
Kubie in U.S. Pat. No. 2,963,391 describes a process for forming a coating designed as an extrusion lubricant wherein the aluminum surface is first treated with an ammonia-laden alkaline solution containing a fatty acid (or equivalent salt or ester thereof) and then baked at about 400.degree. F to form a coating having unknown properties except for lubrication.
Marosi in U.S. Pat. No. 3,849,207 describes and claims a process for treating aluminum and other metals wherein the surface is treated with an alkaline sodium formate solution and then coated with a clear resinous film to form a sepia-colored coating. The nature of the coating formed during treatment in the alkaline sodium formate solution is not described in the reference. However, it has been found that the aluminum surface underlying such a coating is susceptible to water stain or other oxidation in much the same manner as untreated aluminum because the coating is readily wet and penetrated by water or aqueous solutions.
It is against this background that the present invention was developed.